By state law, my child is allowed to attend school until he is age 21 because he is a special needs student. Does the school expense deduction phase out at a particular age?

No. The tax deduction is not determined by the age of a student, but rather if a student is enrolled in either a private school, a public school, or home-schooled from kindergarten through the 12th grade. Since your child is still likely to be designated to a particular grade level, regardless of age, the expenses incurred for school uniforms, tuition, fees, textbooks, instructional materials, and educational supplies are still eligible expenses.